Belt weighers are widely used for measuring a mass of bulk material in motion. It is thought that, at present, approximately 98% of continuous mass measurement of belt-conveyed bulk material is done by belt weighers that use the gravitational measurement principle. Based on the applicant's long period of practice and research in this area, the technological features of known belt weighers are summarized hereinbelow:
1. Known Belt Weighers
Known belt weighers are used to measure the mass of bulk material in motion. Usually, a weighing platform is installed at a certain section of a conveyor belt assembly. That is, a customized belt weigher platform is installed onto the conveyor belt assembly using the original conveyor belt frame and rollers.
Known arrangements of conveyor belt assemblies in use are such that the material being conveyed is disposed on the belt, the weigher platform is disposed under the conveyor belt, the belt being disposed between the material and the weigher platform. As a result, the parameters of the belt greatly affect the accuracy of any bulk material mass measurement.
Due to their unreliable and inaccurate readings, known belt weighers are unsuitable for use in legally binding trade. Thus, most present-day bulk material trades are measured using an age-old waterline method or by using an expensive truck scale. An in-motion belt weigher having improved accuracy would be a great improvement on the present day situation.
2. Structure, Advantages and Disadvantages of Known In-Motion Belt Weighers
A. Multi-Roller Double-Lever Belt Weigher (FIG. 1)
Referring now to FIG. 1, a weighing unit of the type of belt weigher shown consists of two sets of levers facing each other. Each set of levers consists of a pivot 1 (common types include bearing, rubber bearing, knife-edge bearing and flexible tensioner), a frame 2 (normally use channel steel or C-channel steel), weighing rollers 3 (usually two or four sets) and load cells 1. The main advantage of this belt weigher is that the two sets of levers convert the horizontal force on the frame into two equal torques in opposite directions, hence cancelling each other. However, this system has a heavy structure and normally weighs 400-800 kg; the pivot itself in use can cause error as displacement may happen due to the structural deformation of the steel structure which reduce stability; and the length of weighing area is relatively short (only between two or four sets of weighing rollers).
B. Full Suspension Multi-Roller Belt Weigher (FIG. 2)
Referring now to FIG. 2, a weighing unit of the type of belt weigher shown uses three or four load cells 1 to support the weighing platform. Two to four sets of weighing rollers 3 are installed on the weighing platform. The advantages of this system are that it eliminates the errors associated with mechanical hinges, the structure is relatively simple and stability is enhanced compared with the weighing unit design shown in FIG. 1. The disadvantages of this system include an increased number of load cells compared with the design shown in FIG. 1; a requirement for close parameter coordination between the load cells (that is, if the load cell parameters do not closely match, error will occur in the case of uneven and off-centre belt loads); movement of the supporting steel structure of the four load cells can cause interference among the load cells, thus a suspension system using axial and horizontal hinges 5, 6 is needed to connect the load cells and the frame; the structure of the steel frame is heavy (a total of 200-500 kg); and the length of weighing area is relatively short (structural weigh increases significantly if length is increased)
C. Single-Roller Belt Weigher
A weighing unit of this type of belt weigher uses two load cells to support one weighing roller assembly. The advantages of this type of weighing unit include light structure, alleviation of problems associated with mechanical hinges. However, the disadvantages of this type of weighing unit are that it is uneconomical to use two load cells for one weighing roller assembly; there is a requirement for close parameter coordination between the two load cells; interference still occurs when the pivot mounts of the load cells shift, and thus certain flexible connections are still needed; and serial connection of such units becomes unwieldy.
The above-mentioned weighing unit systems are the most commonly used known structures. Other known structures are all based on similar principles and therefore have similar advantages and disadvantages
Overall, known belt weighers cannot meet the requirements of legally-binding trade operation because the problems affecting accuracy and stability of in-motion mass measurement have not been satisfactorily overcome.
The present invention seeks to ameliorate the problems of known belt weigher units through the study of elements affecting accuracy and stability of belt weighers. Compared to known belt weighers, the preferred embodiments of the present invention, which is a belt weighing array system for dispersed material described herein has improved accuracy and reliability as well as simpler structure, therefore making it an ideal candidate for use in trade.
To alleviate the technical problems of known belt weighers, the applicant analysed the main sources of error and concluded that there are three main sources:
A. Load Cell and Computer Instruments
Due to the technological development in load cells and instruments, the error from this area of belt weighers has been reduced to 5-10% of total allowed error and hence is negligible. However, when more than one load cell is used in one weighing unit, parameter coordination problems will arise unless the bulk material on the belt is arranged uniformly.
B. Error Associated with Weighing Frame
The structure of the steel frame has a great impact on accuracy. Stresses caused by welding and deformation of that supporting steel frame can change the force status of the weighing unit frame. Such changes cause interference between hinges and thus result in measurement errors. This is also an important contributing factor to long term reliability problems associated with belt weighers.
C. Effect of Belt Tension
Belt tension is caused by conveyor motors driving the belt. Tension force is affected by the flow of material and it is the major source of error for belt weighing units.
Referring to FIG. 3, a simplified depiction of a belt weigher is shown. In that Figure, the numeral 3′ refers to the fixed roller, numeral 1 refers to the load cell, numeral 3 refers to the weighing roller, numeral 7 refers to the belt and numeral 8 refers to the bulk material carried by the belt. A diagram showing forces acting on each abovedescribed component is presented in FIG. 4 where Fw is the gravitational force of the material, Fc is the gravitational force of the material exerted on the load cell, Fp is the belt tensional force and α is the angle formed by the downward deflection of the weighing roller under the load of the bulk material disposed on the belt. These forces can be expressed in the following formula:Fw−Fc=2Fp×Sin αError-free mass readout is achieved when Fw=Fc, which means that the resulting force of 2Fp×Sin α is the source of error in the mass readout of a weighing unit.
The reason for this error is the relative difference between the height of the weighing roller and the fixed rollers which creates the angle α. There are three factors that contribute to this:                1. The flexing and sinking of weighing frame which causes the angle α. To overcome this, manufacturers normally focus on increasing rigidity which increase the weight of the whole structure;        2. Angle α can increase due to factors such as changes in thickness of the belt and elevation of belt as material adheres to the weighing roller. This is an unavoidable occurrence during the use of belt weighers;        3. The quality of installation of the belt weigher can also contribute to error in that angle α will be positive if the belt weighing unit is not installed in a perfectly horizontal disposition. In addition, belt tension may change in operation due to factors such as the change in material throughput with the result that the error changes.        
The present invention seeks to ameliorate one or more of the abovementioned disadvantages.